This invention relates to the field of non-woven filter elements, and, in particular, filters composed of glass fibers, synthetic short fibers, and polymer fibrids.
The production of a papery product using synthetic fibers is well known. For example, a papery product employing a synthetic fiber such as polyester or nylon or a chemical fiber such as rayon is known. The bonding or matting of such a fiber using a binder, such as a suitable resin, is also known.
A particular type of papery product is that formed mainly of a fiber made of a wholly aromatic polyamide, especially, poly-m-phenylene isophthalamide. Such a product is excellent in its heat-resisting properties and flame retardant properties. For example, in Japanese Patent Publication No. 35-11851, a poly-m-phenylene isophthalamide solution in an amide polar solvent is dispersed into a dispersing medium formed mainly of water to prepare a thin foliated body having a specific configuration, the thin foliated body is then mixed and intertwined with a fiber in water and dried, and the materials are subjected to heat and pressure to prepare a papery product. According to this method, a papery product which is compact in structure and excellent in its electrical insulating property can be obtained, but since the process for preparing the thin foliated body and the papermaking step use a large amount of liquid, this method requires a considerably large amount of energy in the solvent recovery step and the drying step.
To solve this problem, it has been suggested to produce a papery product of high density without employing a papermaking process, which requires the use of a specific binder and a complicated system. It has also been proposed to apply heat and pressure to wholly aromatic polyamide fibers having a low degree of orientation and a low degree of crystallinity (Japanese Laid-open Patent Application No. 52-105975). However, according to this method, since the wholly aromatic polyamide fiber used is inferior in mechanical strength and heat-resistance, the thus obtained papery product inevitably has poor mechanical strength and poor heat-resistance.
U.S. Pat. No. 4,398,995 relates to a papery product composed of a fibrous web, at least part of which is made up of wholly aromatic polyamide fibers having a readily soluble skin layer and a sparingly soluble or insoluble core layer. In forming the web, the skin layer is softened and fuses the polyamide fibers. The papery product may, optionally, include fibers of glass and cellulose acetate. The method of forming papery products in accordance with this patent involves complicated treatment steps to form the core/shell polymer fiber used in the formation of the papery product. This renders the product extremely expensive, and hence impractical, to manufacture.
A combination of glass fibers and aromatic polyamide fibers is disclosed in U.S. Pat. No. 3,920,428. This patent relates to a filter element adapted to remove particulate from the exhaust of an internal combustion engine comprising a non-woven mat of glass fibers fused with aromatic polyamide fibers. More specifically, the aromatic polyamide fibers may be mixtures of fibrids and short fibers as described in U.S. Pat. No. 3,756,908. However, while the materials described in U.S. Pat. No. 3,920,428 may be satisfactory for high temperature applications, it is not a very economically suitable filter under the contemplated circumstances. Moreover, such a filter element is found to exhibit decreased permeability and a low void fraction.
There remains a need, therefore, for an inexpensive and practical filter which is suitable for low temperature applications, and which requires low amounts of energy to provide a strong, yet flexible filter element which has a relatively high void fraction.